This invention relates to controlling the environment surrounding a free fluid stream entering a receiver.
Many organic and inorganic materials have adverse reactions upon contact with gases. For example, atmospheric oxygen may dissolve in hot metal, as when molten metal is exposed to air. The oxygen may oxidize some of the metal and degrade its properties. In production processes, metal often must be poured from one operating or holding container to another. In an example of frequent practice, a quantity of metal is melted in a furnace. The liquid metal is then poured from the furnace into a receiver such as a crucible, mold, ladle, or tundish. If the pouring is performed in air, the liquid stream is susceptible to the solution of, and oxidation by air.
Several methods have been used to protect a molten metal pouring stream. One method of protection is to enclose both containers, such as the furnace and the receiver, in an enclosure which is evacuated of air. This, however, adds considerable capital cost and cumbersomeness to the operation. All actions taken with the enclosed furnace must be by remote control through the wall of the enclosure. The enclosure also interferes with visual observation of the liquid levels in the furnace and receiver which may need to be controlled for proper processing and safety. Operating cost is high because of the power used to operate the vacuum pump, and the inert gas commonly used to backfill the enclosure to allow the molten metal to be removed from the receiver.
Another method is to pour the molten metal from the furnace to the receiver through a tube or shroud. However this is inconvenient and does not provide complete protection. Because the furnace is usually tilted for pouring, a rigid tube or shroud of fixed dimensions cannot be adjusted to the changing distance from the furnace spout and the receiver opening. Before pouring begins, the tube or shroud interior is ordinarily occupied by air which comes into contact with the liquid stream when pouring begins. In addition, the tube or shroud interferes visual observation of the liquid level through the receiver opening.
In another method, liquified inert gas, such as liquid nitrogen, is introduced into the receiver so that when liquid metal is poured into the receiver, the liquified gas can float on the surface of the liquid metal. The liquefied gas vaporizes into gas which displaces air from the receiver volume and rises from the receiver at least partially protecting the pouring liquid stream from exposure to air. This method adds considerable operating cost in that a considerable quantity of liquified gas is used. Further, the method presents a safety hazard in that liquified gas can be trapped under the molten metal, and subsequent rapid gasification of the trapped liquified gas can produce an explosion.
The problem of air oxidation of hot pouring metal is especially important in the production of metal powder. Metal powder is used to manufacture many metal articles by forming the powder under pressure and temperature into a desired shape. This powder metallurgy method is preferred for alloys which are difficult to cast or difficult to form mechanically. The powder metallurgy method may also be used to imbue a finished article with unusual properties related to the fine powder or the rapid speed of solidification of the powder.
Metal powder is produced by melting a quantity of metal and creating a stream of the molten metal which is atomized into droplets which solidify into powder. The atomization is usually carried out by directing a stream of molten metal through high-velocity jets of water, oil, or inert gas. While it is possible to tilt a furnace to pour a molten metal stream through atomizing jets, the efficiency of this process is compromised because it is difficult to control geometric relationships between the stream and the atomizing jets. Consequently solid metal is usually melted in a furnace, and the liquid metal is poured into a geometrically fixed tundish from which a stream of molten metal is directed to the atomizing jets.
What is needed is an inexpensive, unobtrusive method of providing a controlled protective environment for the receiver opening and the free stream entering the receiver opening.